There are many instances where it is desirable to be able to diagnose intermittent spontaneous cardiac arrhythmias in ambulatory patients. Frequently faintness, syncope, and tachyarrhythmia palpitation symptoms cannot be induced and observed by the physician in tests conducted in a clinic. For many years, such patients have been equipped with external ECG monitoring systems, e.g., the patient-worn, real time Holter monitors, that continuously sample the ECG from skin electrodes and record it over a certain time period. Then, the ECG data must be analyzed to locate evidence of an arrhythmia episode from which a diagnosis can be made.
As described in commonly assigned U.S. Pat. No. 5,312,446 and in U.S. Pat. No. 4,947,858, both incorporated herein by reference, the externally worn ECG recorders have inherent limitations in the memory capacity for storing sampled ECG and EGM data. Cost, size, power consumption, and the sheer volume of data over time have limited real time external Holter monitors to recording 24 hour segments or recording shorter segments associated with arrhythmias that are felt by the patient who initiates storage.
The use of the externally worn Holter monitor coupled with skin electrodes is also inconvenient and uncomfortable to the patient. The skin electrodes can work loose over time and with movement by the patient, and the loose electrodes generates electrical noise that is recorded with the EGM signal and makes its subsequent analysis difficult. It has long been desired to provide an implantable monitor or recorder that is hardly noticeable by the patient and provides the capability of recording only EGM data correlated with an arrhythmia episode that is automatically detected.
Over the last 40 years, a great many IMDs have been clinically implanted in patients to treat cardiac arrhythmias and other disorders including implantable cardioverter/defibrillators (ICDs) and pacemakers having single or dual chamber pacing capabilities, cardiomyostimulators, ischemia treatment devices, and drug delivery devices. Recently developed implantable pacemakers and ICDs have been provided with sophisticated arrhythmia detection and discrimination systems based on heart rate, the morphology and other characteristics of the atrial and ventricular EGM and other characteristics of the EGM. Most of these IMDs employ electrical leads bearing bipolar electrode pairs located adjacent to or in a heart chamber for sensing a near field EGM or having one of the electrodes located on the IMD housing for sensing a far field, unipolar EGM. In either case, the near field or far field EGM signals across the electrode pairs are filtered and amplified in sense amplifiers coupled thereto and then processed for recording the sampled EGM or for deriving sense event signals from the EGM.
In current IMDs providing a therapy for treating a cardiac arrhythmia, the sense event signals and certain aspects of the sampled EGM waveform are utilized to automatically detect a cardiac arrhythmia and to control the delivery of an appropriate therapy in accordance with detection and therapy delivery operating algorithms. In such cardiac IMDs providing pacing or cardioversion/defibrillation therapies, both analog and digital signal processing of the EGM is continuously carried out to sense the P-wave and/or R-wave events and to determine when a cardiac arrhythmia episode occurs. For example, a digital signal processing algorithm is employed to distinguish various atrial and ventricular tachyarrhythmias from one another. When a tachyarrhythmia episode is detected, at least selected EGM signal segments and sense event histogram data or the like are stored on a FIFO basis in internal RAM for telemetry out to an external programmer at a later time. Many of these IMDs are also capable of being operated to sample the EGM and transmit real time EGM data of indefinite length via uplink telemetry transmissions to the external programmer when a real time telemetry session is initiated by the medical care provider using the programmer.
Implantable cardiac monitors have also been developed and clinically implanted that employ the capability of recording cardiac EGM data for subsequent interrogation and uplink telemetry transmission to an external programmer for analysis by a physician. The recorded data is periodically telemetered out to a programmer operated by the medical care provider in an uplink telemetry transmission during a telemetry session initiated by a downlink telemetry transmission and receipt of an interrogation command.
The MEDTRONIC.RTM. Reveal.TM. insertable loop recorder is a form of implantable monitor that is intended to be implanted subcutaneously and has a pair of sense electrodes spaced apart on the device housing that are used to pick up the cardiac far field EGM which in this case is also characterized as a "subcutaneous ECG". The Reveal.TM. insertable loop recorder samples and records one or more segment (depending on the programmed operating mode) of such far field EGM or subcutaneous ECG signals when the patient feels the effects of an arrhythmic episode and activates the recording function by applying a magnet over the site of implantation. For example, the storage of a programmable length segment of the EGM can be initiated when the patient feels faint due to a bradycardia or tachycardia or feels the palpitations that accompany certain tachycardias. The memory capacity is limited, and so the segments of such EGM episode data that are stored in memory can be written over with new EGM episode data when the patient triggers storage and the memory is full. The most recently stored segment or segments of episode data is transmitted via an uplink telemetry transmission to an external programmer when a memory interrogation telemetry session is initiated by the physician or medical care provider using the programmer. Aspects of the Reveal.TM. insertable loop recorder are disclosed in commonly assigned PCT publication WO98/02209.
More complex implantable monitors and pacemaker IPGs of this type but having more electrodes arranged in a planar array on the device housing are disclosed in commonly assigned U.S. Pat. No. 5,331,966, incorporated herein by reference. Three electrodes are employed to provide a pair of orthogonal sensed EGM or "subcutaneous ECG" signals at the subcutaneous implantation site. A lead can be employed in a disclosed pacemaker embodiment to locate a bipolar electrode pair in a heart chamber to provide an additional near field EGM sense signal from which the P-wave or R-wave can be sensed (depending on the location of the bipolar electrode pair) and through which pacing pulses can be applied to the atrium or ventricle. Recording of the near field and far field EGM episode data can be invoked automatically by detection of a bradycardia or satisfaction of tachyarrhythmia detection criteria or can be manually commenced by the patient using an external limited function programmer or can be commenced by the physician using a full function programmer.
In all of these IMDs having a cardiac monitoring function, the cardiac EGM is continually sensed and sampled in such monitors and recording of EGM episode data is triggered in a variety of ways. The relatively inexpensive and simple to implant Reveal.TM. insertable loop recorder has been favorably compared to the "black box" of an aircraft by physicians that have prescribed its implantation and use in a number of patients. Recordings of EGM episode data triggered by the patient using the relatively simple Reveal.TM. insertable loop recorder have proven to be of great value in diagnosing the causes of symptoms felt by the patients and in prescribing the implantation and programming of more complex therapy delivery IMDs, e.g., multi-programmable physiologic DDDR pacemakers and single and dual chamber ICDs.
However, many times patients are either unaware of "silent" cardiac arrhythmias or are asleep or fail to activate the recording function when they recover from syncope (i.e., have fainted) when bradycardias and tachyarrhythmias occur, and so the accompanying EGM episode data is not recorded. It is desired to be able to automatically detect an arrhythmia and to initiate recording of the EGM data without having to rely upon the patient as disclosed in the above-incorporated '966 patent. But, the subcutaneous location environment of the sense electrode pair or pairs on the device housing is relatively noisy due to electromyographic signals generated by adjacent muscle groups that are exercised by the patient. Limb and trunk movements or even breathing can generate noise spikes that are superimposed upon the far field EGM signal and can make it appear to reflect a higher heart rate than the actual heart rate. The electromyographic noise level is not as pronounced in relation to the EGM signal level when bipolar sense electrode pairs located in or close by the atrium and ventricle are employed as is typically the case with bipolar implantable pacemakers and ICDs. Consequently, it is usually possible to filter out such noise in the sense amplifiers of such IMDs. And, the patient implanted with the Reveal.TM. insertable loop recorder can be instructed to assume a quiet body state when he/she initiates recording. Moreover, even if noise artifacts are recorded, they are recorded within EGM episode data that does represent an arrhythmia felt by the patient.
In this context, if an implantable monitor of this type is implemented with an automatic arrhythmia detection function, it will automatically commence the recording of the EGM episode data when noise artifacts are superimposed on the EGM signal being monitored and if the detection algorithm mistakenly detects an arrhythmia as a consequence. On the other hand, sometimes such noise is present during an actual arrhythmia of interest that is correctly detected, triggering the recording of EGM episode data that is of interest but is also noisy. Due to the limited memory capacity, the EGM data episode that is corrupted by noise signals will be written over earlier recorded EGM episode data that is either also corrupted or is relatively noise free and either does or does not actually represent an arrhythmia episode of interest. The physician may find that the EGM episode data that is later uplink transmitted and displayed by the programmer is simply corrupted and of no value in diagnosing the patient's cardiac condition. Then, the physician may have to program the IMD detection algorithm differently (if it is possible to do so) or have to program the automatic detection and recording capability off and rely upon the patient to trigger the recording of EGM episode data when the onset of an arrhythmia is felt.
Thus, a need exists for a simple system for providing the automatic detection and recording functions in an implantable monitor of this type while avoiding filling the memory registers with EGM episode data that is noise corrupted.